Crystalline solar cells in recent years have a pn junction type photodiode structure, generally provided with a p-type semiconductor on one side of a substrate and an n-type semiconductor on the other side. Most of such solar cells have had a configuration in which a diffusion layer of a conduction type (n type) opposite to the conduction type (p type) of the semiconductor substrate is provided on the light-receiving side, and an SiNx film as an anti-reflection film is formed on the diffusion layer.
FIG. 1 shows an example of the sectional configuration of a solar cell according to the related art in which a p-type semiconductor substrate is used.
As shown in FIG. 1, a thin diffusion layer 103 of a conduction type (n type) opposite to the conduction type of the p-type semiconductor substrate 104 is provided on the light-receiving surface side, and a silicon nitride (SiNx) film as an anti-reflection film 102 is formed thereon. In addition, electrodes 101 for collecting photo-excited carriers are provided on the anti-reflection film 102 at an interval of several millimeters (an interval of about 0.1 to 5 mm). On the other hand, current collection electrodes 107 are provided on the back side at an interval of several millimeters. As the material of these electrodes 101 and 107, silver (Ag) has often been used, from the viewpoint of conductivity. In addition, since the solar cells have to be fabricated at low cost, it has been a common practice to form the electrodes by applying a conductive metal-containing paste in the shape of electrodes by a printing method, followed by sintering at a high temperature.
In addition, those regions on the back side (p-type region) of the p-type semiconductor substrate 104 which are other than the regions of the electrodes 107 are protected (passivated) by a passivation film 105 composed of a silicon oxide (SiO2) film. Such a structure wherein an SiO2 film is formed as a passivation film on the back side is described in, for example, JP-A H09-097916.
Besides, an SiO2 film is commonly formed by thermal oxidation. In this connection, JP-A H08-078709 discloses a method in which an SiO2 film showing a good passivation effect is formed by a chemical treatment. Further, JP-A 2003-347567 and JP-A 2004-006565 each describe a method in which an SiO2 film is formed by coating.
In addition, WO 2008/065918 discloses that an aluminum oxide film is also effective as a passivation film for p-type regions.
Besides, JP-A H10-229211 discloses a configuration in which a silicon nitride film is formed as a passivation film on a semiconductor substrate.
Meanwhile, in recent years, the use of n-type semiconductor substrates as means for enhancing the efficiency of crystalline silicon solar cells has been investigated (see, for example, JP-A 2005-327871). In the cases of using such an n-type semiconductor substrate, further enhancement of photoelectric conversion efficiency is being desired.